1. Field of the Invention
The present invention generally relates to a transceiver and a system for high speed interface. More particularly, the present invention relates to a transceiver and a high speed interface system: actively selecting an active termination circuit of which input impedance is matched or approximately matched to characteristic impedance of a transmission line, and connecting the active termination circuit to the transmission line; selectively connecting a self-terminated driver circuit of which output impedance is matched or approximately matched to characteristic impedance of the transmission line.
2. Description of the Prior Art
According to continuous increase of data transmission speed and clock speed in computer system, reflection wave resulting from impedance mismatching also increases. As a result, it is difficult to transmit and receive precisely data signals in high speed interface. In general, output impedance of transmitter and input impedance of receiver do not match with characteristic impedance of transmission line regardless of kinds of transmission line connected to transceiver. To overcome this problem, the conventional art uses a receiver and an open-drain driver for low swing high speed, and a method wherein single off-chip parallel termination resister of which impedance is matched to a transmission line is connected to a termination line of the transmission line.
FIG. 1 is a block diagram illustrating a conventional high speed interface system including a plurality of transceivers. A plurality of transceivers 10, 20, 30 and 40 are connected to a transmission line. In a first transceiver 10, a transmitter 12 and a receiver 15 are all illustrated, but in the rest transceivers 20, 30 and 40, receivers 25, 35 and 45 are just illustrated for the convenience of explanation. This system inevitably has parasitic capacitance cp1, cp2, . . . , cpN in each stub channel and each receiver 15, 25, 35, 45. The parasitic capacitance elongates rising and falling time of transmission signals, and lowers limits operating frequency, thereby requiring extra parallel termination resistor. As a result, the system has complicated design and high fabrication cost.
As rising and falling time of transmission signals is shortened and operating frequency increases, reflection wave resulting from discontinuous point on channel including input units of receivers 15, 25, 35, 45 increases. However, if output impedance of transmitter 12 and input impedance of receivers 15, 25, 35, 45 are not matched to characteristic impedance of transmission lines, the reflection wave is not sufficiently absorbed. As a result, signal integrity of transmission signals received by receivers 15, 25, 35, 45 are remarkably degraded. Accordingly, in order to improve signal integrity, it is required to provide impedance matching or approximate matching simple and convenient in high speed interface system including a plurality of transceivers.